Information recording medium and its production method, and optical information recording reproducing apparatus

ABSTRACT

An information recording medium has a recording portion where three-dimensional information can be recorded. The recording portion includes at least one particle-containing layer (recording layer or a recording auxiliary layer) containing particles absorbing at least a part of light having a predetermined wavelength and substantially transparent to a recording light and a reproducing light having a wavelength longer than the predetermined wavelength and a particle holding material substantially transparent to the recording and reproducing lights. The mean particle size of the particles is preferably shorter than the wavelengths of the recording and reproducing lights.

TECHNICAL FIELD

The present invention relates to an information recording medium capableof three-dimensionally recording information and a method for producingthe same, and an optical information recording/reproducing apparatus,and more particularly to an information recording medium capable ofrecording with high sensitivity and at high speed and a method forproducing the same, and an optical information recording/reproducingapparatus.

BACKGROUND ART

One of the conventional information recording medium capable ofthree-dimensionally recording information is shown in FIG. 8 (YoshimasaKawata et al.: Three-dimensional optical memory using an organicmultilayered recording medium; Optics Japan 2000, Proceedings, p. 95-96,7 p B12 (2000)). In this information recording medium, on a glasssubstrate 104, recording layers 101 a to 101 d made of urethane-ureacopolymer material that is a photon-mode recording material, andintermediate layers 102 a to 102 c made of a PVA (polyvinyl alcohol)film and a PMMA (polymethyl methacrylate) film are laminatedalternately.

Laser light 108 is focused (i.e., irradiation with convergent light 107)on a desired recording layer of the recording layers 101 a to 101 d ofthis information recording medium by an objective lens 106, wherebyinformation can be recorded. The laser light 108 used herein is pulselaser light having a pulse width of about 100 femtoseconds and anextremely high peak power. By focusing such pulse laser light on therecording layers 101 a to 101 d, information is recorded on therecording layers 101 a to 101 d by using the two-photon absorption thatis one of the non-linear absorption phenomena. Specifically, in aportion where a power density of the convergent light 107 is high (in alight-focusing point) in a region that is irradiated with the convergentlight 107 in the recording layers 101 a to 101 d, the two-photonabsorption occurs. In this state a phenomenon that looks as if theportion were irradiated with light with a half wavelength of thewavelength of actually irradiated light occurs, and an information bit105 is written. Furthermore, on the information bit 105, low power lightis focused, and light reflected therefrom is detected by a photodetector(not shown) via the objective lens 106, whereby signals can bereproduced. In this information recording medium, since a plurality ofrecording layers are laminated in the direction of an optical axis ofthe objective lens (z direction), information can be recordedthree-dimensionally and the recording capacity is increased.

However, there has been a problem in the above-mentioned conventionalinformation recording medium that the recording sensitivity of therecording layer was not good. Accordingly, in the case of a recordingmethod in which one information bit is formed by one pulse, it isnecessary to use a femtosecond laser having an extremely large peakpower (about 100 kW) as a light source, and thus the structure of thelight source becomes complicated. Furthermore, there has been anotherproblem in that in the case where a light source with a smaller peakpower is used, pulse irradiation at the same place is required to berepeated several times (for example, from several tens to severalthousands times) (i.e., since a photon-mode recording material is usedfor a recording layer, accumulated recording of refractive index changeis possible), whereby the speed of writing information is reduced.

DISCLOSURE OF THE INVENTION

The information recording medium of the present invention a recordingportion capable of recording information three-dimensionally. Therecording portion comprises at least one particle-containing layercomprising: particles that absorb at least a part of light with apredetermined wavelength; are substantially transparent to recordinglight and reproducing light with wavelengths longer than thepredetermined wavelength; and have an absorption rate with respect tothe light with the predetermined wavelength that is higher than theabsorption rate with respect to the recording light and the reproducinglight, and a particle-holding material that is substantially transparentto the recording light and the reproducing light.

The method for producing an information recording medium of the presentinvention includes forming a coating containing particles and aparticle-holding material; and applying the coating so as to form aparticle-containing layer.

The optical information recording/reproducing apparatus of the presentinvention includes a light source for emitting recording light; a lightsource for emitting reproducing light; an objective lens for focusingthe light emitted from the light source on the information recordingmedium; and a photodetector for detecting light reflected by theinformation recording medium. By using the change in the opticalconstant of the recording portion of the information recording medium,an information bit is recorded on the recording portionthree-dimensionally.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view showing a cross-sectional configuration of aninformation recording medium and explaining a state in which informationis recorded/reproduced according to Embodiment 1 of the presentinvention; and FIG. 1B is an enlarged view showing a recording layer ofthe information recording medium shown in FIG. 1A.

FIGS. 2A to 2D are cross-sectional views showing each step of producingan information recording medium according to Embodiment 1 of the presentinvention.

FIG. 3 is a schematic view showing an optical head of an opticalinformation recording/reproducing apparatus according to Embodiment 1 ofthe present invention.

FIG. 4 is a graph showing a spectral transmittance curve of therecording layer of the information recording medium according toEmbodiment 1 of the present invention.

FIG. 5A is a view showing a cross-sectional configuration of aninformation recording medium and explaining a state in which informationis recorded/reproduced according to Embodiment 2 of the presentinvention; and FIG. 5B is an enlarged view showing a recording layer andan auxiliary recording layer of the information recording medium shownin FIG. 5A.

FIG. 6A is a view showing a cross-sectional configuration of aninformation recording medium and explaining a state in which informationis recorded/reproduced according to Embodiment 3 of the presentinvention; and FIG. 6B is an enlarged view showing a recording layer ofthe information recording medium shown in FIG. 6A.

FIG. 7 is a graph showing the relationship between a pulse width of therecording laser light and both an energy threshold value and a peakpower threshold value at the time of recording one information bit onthe information recording medium of the Example of the presentinvention.

FIG. 8 is a view showing a cross-sectional configuration of aconventional information recording medium and explaining a state inwhich signals are recorded/reproduced.

BEST MODE OF CARRYING OUT THE INVENTION

According to the information recording medium of the present invention,since the recording sensitivity is enhanced, it is possible to form oneinformation bit by one pulse even if laser light having an extremelyhigh peak power, which was required in the conventional example, is notused. Thus, the present invention can provide an information recordingmedium capable of recording information with high sensitivity at highspeed. Note here that, throughout the description, the term“substantially transparent to recording light and reproducing light”means that almost all light components of the recording light and thereproducing light, except for scattered light components, are allowed tobe transmitted without being absorbed. Specifically, for example, it ispreferable that the transmittance with respect to oneparticle-containing layer is 95% or more, and more preferably 99% ormore.

In the information recording medium of the present invention, it ispreferable that the average particle-size of the particles is shorterthan the wavelength of the recording light and the wavelength of thereproducing light. It is more preferable that the average particle-sizeof the particles is shorter than a quarter of the wavelength of therecording light and a quarter of the wavelength of the reproducinglight. This is advantageous because, for example, in the case where aplurality of particle-containing layers are included in the recordingportion, diffraction loss or scattering loss is suppressed so as toprevent the optical loss. Note here that in this case, since a part ofparticles is aggregated, particles (aggregate) whose apparentparticle-size is larger than the wavelength of the recording light andthe reproducing light may be present. Furthermore, even when theparticles are aggregated, it is preferable that the apparentparticle-size of the particles in an aggregated state is shorter thanthe wavelength of the recording light and the reproducing light.

In the information recording medium of the present invention, theparticle-containing layer may be a recording layer, and theparticle-holding material may have an optical constant that changes at apredetermined temperature. In this case, the recording portion includesa plurality of the recording layers and the plurality of the recordinglayers may be laminated via intermediate layers that are substantiallytransparent to the recording light and the reproducing light so as toform a multilayered structure.

Furthermore, in the information recording medium of the presentinvention, the recording portion further may include a recording layerand the particle-containing layer may be an auxiliary recording layerdisposed in contact with the recording layer. In this case, amultilayered structure may be obtained by providing a plurality oflaminate bodies composed of the recording layer and the auxiliaryrecording layer, and by providing an intermediate layers that aresubstantially transparent to the recording light and the reproducinglight between the laminated bodies that are adjacent to each other.

Furthermore, in the information recording medium of the presentinvention, the particle-containing layer may be a recording layer, andthe particle-holding material may have an optical constant that changesat a predetermined temperature, and an entire recording portion may beformed as one recording layer.

In the information recording medium of the present invention, aprotective layer that is substantially transparent to the recordinglight and the reproducing light is provided at the side of the recordingportion on which light is incident. It is advantageous because therecording portion can be protected from damage or dust, etc.

Furthermore, it is preferable that the protective layer or theintermediate layer is made of the same material as that of theparticle-holding material. It is advantageous because an excellentinterface between the particle-containing layer and the protective layeror the intermediate layer can be formed and also the materialmaintenance can be facilitated. Herein, the term “the same material”means the same kind of material. Thus, for example, resins each havingdifferent molecular weights due to the difference in the polymerizationdegree are regarded as the same material.

In the information recording medium of the present invention, it ispreferable that the difference between the refractive index of theparticle-holding material and the refractive index of the particles isor less. This is advantageous because the scattering loss can besuppressed.

In the information recording medium of the present invention, it ispreferable that the particles include an inorganic material or asemiconductor material. When the particles are formed of inorganicmaterials, the particle-containing layer can be stabilized. When theparticles are formed of semiconductor materials, since the transmittancecan be reduced rapidly in a certain range of wavelengths, the materialis suitable for recording information by using a nonlinear absorptionphenomenon. In such semiconductor materials, it is preferable that anenergy gap is 2.5 eV or more and 8.3 eV or less. For example, it ispreferable that the material include at least one selected from thegroup consisting of zinc oxide, tin oxide, zinc sulfide, titanium oxide,tungsten oxide, strontium titanate, silicon carbide, indium oxide andcadmium sulfide. This is advantageous because since the wavelength ofthe reproducing light can be set to be 500 nm or less, so that thereproducing resolution and recording density can be improved.

In the information recording medium using a plurality ofparticle-containing layers as a recording layer or an auxiliaryrecording layer, it is preferable that the particle-containing layercontains 1 wt. % or more and 95 wt. % or less of the particles. In thiscase, it is advantageous because by setting the content of the particlesto be 1 wt. % or more, the recording sensitivity can be improved to thepractical level, and by setting the content of particles to be 95 wt. %or less, the optical loss such as scattering of light, can be reduced toa practical level.

In the information recording medium in which the particle-containinglayer is a recording layer and the whole recording portion is onerecording layer, it is preferable that the particle-containing layercontains 0.3 wt. % or more and 10 wt. % or less of the particles. Inthis case, it is advantageous because by setting the content ofparticles to be 0.3 wt. % or more, the recording sensitivity can beimproved to the practical level, and by setting the content of theparticles to be 10 wt. % or less, the optical loss such as scattering oflight can be reduced to a practical level.

Furthermore, in the information recording medium of the presentinvention, it is preferable that the particle-holding material is formedof a resin. It is advantageous because the particle-containing layer canbe formed easily.

According to a method for producing the information recording medium ofthe present invention, the information recording medium of the presentinvention can be produced easily and at low cost.

In the method for producing the information recording medium of thepresent invention, it is preferable that the method further includesforming an intermediate layer by applying the coating containingmaterials substantially transparent to recording light and reproducinglight, wherein in the step of forming the particle-containing layer, amaterial having an optical constant that changes at a predeterminedtemperature is used as the particle-holding material, and the step offorming the particle-containing layer and the step of forming theintermediate layer are repeated alternately a predetermined number oftimes. Thus, the particle-containing layer can be a recording layer, anda laminated bodies, in which a plurality of the recording layers arelaminated, can be produced easily and at low cost.

In the method for producing the information recording medium of thepresent invention, the method further may include forming anintermediate layer by applying a coating containing a material that issubstantially transparent to the recording light and the reproducinglight; and forming a recording layer by applying the coating containinga material having an optical constant that changes at a predeterminedtemperature; wherein the step of forming the particle-containing layer,the step of forming the recording layer, and the step of forming theintermediate layer are repeated in a predetermined order andpredetermined number of times periodically. According to this method,the information recording medium including a plurality of the recordinglayers and the auxiliary recording layers composed of theparticle-containing layer can be produced easily and at low cost.

According to the optical information recording/reproducing apparatus ofthe present invention, information can be recorded on the informationrecording/reproducing medium of the present invention with high densityand at high speed.

In the optical information recording/reproducing apparatus, it ispreferable that the light source for emitting recording light is a pulselaser light source and a pulse width is in the range from 100femtoseconds to 10 nanoseconds. It is more preferable that the pulsewidth is in the range from 1 picosecond to 100 picoseconds. This isadvantageous because the recording sensitivity can further be enhanced.

In the optical information recording/reproducing apparatus of thepresent invention, it is preferable that the wavelength of the lightsource for emitting reproducing light is shorter than the wavelength ofthe light source for emitting recording light. This is advantageousbecause reproducing resolution and the recording density can beimproved.

In the optical information recording/reproducing apparatus of thepresent invention, it is preferable that the information bit is recordedon the recording portion of the information recording medium by using anonlinear absorption phenomenon. It is preferable that the nonlinearabsorption phenomenon includes the two-photon absorption or themultiphoton absorption.

In the optical information recording/reproducing apparatus of thepresent invention, it is preferable that the information bit is recordedthree-dimensionally on the recording portion of the informationrecording medium in the order in which the recording light does not passthrough the information bit that has already been recorded on therecording portion. For example, it is preferable that the informationbit is recorded sequentially in the order from the position distant fromthe objective lens to the position near the objective lens in therecording portion of the information recording medium. This isadvantageous because by recording information bits in such an order, itis possible to reduce stray light such as scattered light, anunnecessary refractive light, and the like.

In the optical information recording/reproducing apparatus of thepresent invention, it is preferable that one light source is shared forthe light source for emitting the recording light and the light sourcefor emitting the reproducing light. This is advantageous because thenumber of light sources is one, and thus the configuration can besimplified.

Hereinafter, the embodiments of the present invention will be explainedwith reference to drawings.

Embodiment 1

An information recording medium and a method for producing the same, andan optical information recording/reproducing apparatus according toEmbodiment 1 of the present invention will be explained with referenceto drawings. FIGS. 1A and 1B show a cross-sectional configuration of theinformation recording medium and a state in which information isrecorded and reproduced in the embodiment of the present invention.FIGS. 2A to 2D are cross-sectional views showing each step of producingan information recording medium according to this embodiment. FIG. 3shows a schematic configuration of an optical head of an opticalinformation recording/reproducing apparatus according to thisembodiment. FIG. 4 shows a spectral transmittance curve of the recordinglayer of an example of the information recording medium according tothis embodiment.

As shown in FIG. 1A, in the information recording medium of thisembodiment, on a substrate 9, a recording portion 3 and a protectivelayer 4 are formed. In the recording portion 3, a plurality of recordinglayers (particle-containing layers) are laminated via intermediatelayers. That is to say, in the recording portion 3, from the side of thesubstrate 9, a recording layer 1 a, an intermediate layer 2 a, arecording layer 1 b, . . . , a recording layer 1 e, an intermediatelayer 2 e, and a recording layer 1 f are laminated sequentially. In theinformation recording medium of this embodiment, by including aplurality of recording layers in the recording portion 3, in addition totwo-dimensional recording of information, three-dimensional recordingcapable of recording information by superimposing a plurality of layersin the thickness direction can be achieved. Note here that hereinafter,an arbitrary recording layer in the recording layers 1 a to 1 f will bereferred to as a recording layer 1 and an arbitrary intermediate layerin the intermediate layers 2 a to 2 e will be referred to as anintermediate layer 2.

As shown in FIG. 1A, in the information recording medium of thisembodiment, at the time of recording and reproducing information, lightis incident on the side of the protective layer 4. At the time ofrecording information, laser light 8 is allowed to be focused on any oneof the recording layers 1 a to 1 f by the objective lenses (lens pair 6a and 6 b) (converging light beam 7), and an information bit 5 isformed. At the time of reproducing information, laser light 8 is allowedto be focused on a desired layer of the recording layers 1 a to 1 f bythe objective lenses (lens pair 6 a and 6 b) (converging light beam 7)so as to reproduce information by light reflected by the information bit5.

FIG. 1B is an enlarged view showing the recording layer 1 (a region 12shown in FIG. 1A). As shown in FIG. 1B, the recording layer 1 is formedof a particle-holding material (binder) 10 in which particles 11 aredispersed.

The particles 11 are made of a material that is substantiallytransparent to recording light and reproducing light, and absorbs atleast a part of light with a predetermined wavelength. Thispredetermined wavelength is shorter than the wavelength of the recordinglight (recording wavelength) and the wavelength of the reproducing light(reproducing wavelength), and the absorption rate of the particles 11with respect to the light with a predetermined wavelength is higher thanthe absorption rate of the particles 11 with respect to the recordinglight and reproducing light. Furthermore, this predetermined wavelengthis included in the predetermined range of wavelengths including thewavelength of the absorption spectrum of the particles 11 (in the rangeof the wavelength in which the transmittance of the particles isreduced) and the absorption rate of the particles 11 with respect to thelight of this predetermined wavelength is preferably 5% or more and morepreferably 20% or more per recording layer. Furthermore, it ispreferable that the average particle-size of the particles 11 is smallerthan the recording wavelength and the reproducing wavelength. This isbecause optical loss due to the diffraction loss of the recording lightand reproducing light by the particles 11 is suppressed. Furthermore, bymaking the particle-size of the particles 11 to be a quarter of therecording wavelength and the reproducing wavelength, since not only thediffraction loss but also the scattering loss can be suppressed, theoptical loss of the recording light and the reproducing light can bereduced.

The particle-holding material 10 is made of a material that issubstantially transparent to the recording light and the reproducinglight. Furthermore, since the particle-holding material 10 is used forrecording materials, it is formed of, for example, a material whoseoptical constant changes at predetermined temperature, for example, amaterial whose refractive index changes (e.g., occurrence of thermaldeformation, etc.) at a predetermined temperature. For theparticle-holding material 10, for example, PMMA, a photopolymer resin,an ultraviolet curable resin, polyester, or the like, can be used.

It is preferable that the recording layer 1 includes 1 wt. % to 95 wt. %of the particles 11. When the thickness of one recording layer is thin,e.g., about 1 μm, if about 1 wt. % of the particles 11 are present, therecording sensitivity can be improved and if about 95 wt. % of theparticles 11 are present, the optical loss due to light scattering canbe suppressed. Furthermore, as mentioned below, since theparticle-holding material 10 functions as a recording material, it ispreferable that 30 wt. % or more of the particle-holding materials 10are present. Therefore, it is preferable that the particles 11 arecontained in the range of 70 wt. % or less. Furthermore, in order toachieve a sufficient recording sensitivity, the particles 11 arecontained in the range of 30 wt. % or more.

By setting the difference between the refractive index of the particles11 and the refractive index of the particle-holding material 10 to be0.5 or less, it is possible to suppress the scattering loss of incidentlight, thus improving the efficiency in using light.

The intermediate layer 2 is substantially transparent to the recordinglight and the reproducing light. For example, PMMA, a photopolymerresin, an ultraviolet curable resin, polyester, or the like, can beused. The intermediate layer 2 may be formed of the same material asthat of the particle-holding material 10 of the recording layer 1.

The substrate 9 can be formed of, for example, polycarbonate, PMMA, anorbornene resin (for example, “ARTON” (JSR Corporation)), a cycloolefinresin (for example, “ZEONEX” (ZEON CORPORATION, JAPAN)), or the like.The protective layer 4 can be formed of a transparent material, and forexample, polycarbonate, PMMA, a norbornene resin (for example, “ARTON”(JSR Corporation)), a cycloolefin resin (for example, “ZEONEX” (ZEONCORPORATION, JAPAN)), or the like, can be used.

Next, recording of information on the information recording mediumaccording to this embodiment will be explained.

The particles 11 and the particle-holding material 10 are substantiallytransparent to the recording light and the reproducing light. Therefore,in the information recording medium having a multilayered recordinglayer in this embodiment, the recording light and the reproducing lightcan reach the layer (a recording layer 1 a in FIG. 1) that is the mostdistant from the surface on which light is incident, without muchattenuation, enabling information to be recorded and reproducedthree-dimensionally.

When light with a relatively high peak power is focused on the recordinglayer 1 as the recording light, a nonlinear absorption phenomenon suchas the two-photon absorption, or multiphoton absorption, is induced at ahigh rate in the focal point with high optical power density (photondensity). Note here that the nonlinear absorption phenomenon hereindenotes a phenomenon in which the absorption sensitivity of therecording layer 1 is not proportional to the energy of light. An exampleof the nonlinear absorption phenomenon includes: the case where if theabsorption sensitivity has a threshold value (in particular, in thisembodiment, the threshold value is relatively large (for example, theenergy threshold value: 10 pJ/μm² or more)), the two-photon absorptionin which the absorption sensitivity is proportional to almost the squareof the energy of light, or the multiple photon absorption in which theabsorption sensitivity is proportional to almost the n-th power (n is aninteger of 3 or more) of the energy of light occurs; the case where thetwo-photon absorption or the multiple photon absorption triggers plasma,and the combination thereof, and the like. For example, in the casewhere the two-photon absorption occurs in the recording layer 1 in thefocal point of the recording light, the recording layer 1 in the focalpoint absorbs two-photons at the same time, and it looks as if therecording layer were irradiated with the light with a half wavelength ofthe recording wavelength. In this case, if the particles 11 are formedof materials that absorb at least a part of the light with a halfwavelength of the recording wavelength of the particles 11 (thepredetermined wavelength is set to be half of the recording wavelength),the particles 11 absorb the recording light at the focal point of therecording light. Heat caused by the light absorption of the particles 11raises the temperature of the recording layer and reaches thepredetermined temperature, and the optical constant of theparticle-holding material 10 changes so as to form information bit 5.Thus, it is thought that at the time of recording, the optical constantof the particles 11 does not change for themselves but they act as acatalyst for changing the optical constant of the particle-holdingmaterial 10 so as to form the information bit 5.

In order to induce the nonlinear absorption phenomenon in the focalpoint of the recording light, conventionally, it was necessary to use alaser light with an extremely high peak power or to repeat irradiationat one place multiple times in the case where the peak power is not sohigh. However, according to the information recording medium of thisembodiment, even if the peak power is about several tens to severalhundreds mW, such a nonlinear absorption phenomenon can be induced.Furthermore, even if the pulse laser light does not have such a highpeak power, one information bit 5 can be formed by one pulse. This isthought because light is absorbed by the particles 11, thereby theeffect unique to the particles, that is, excellent quantum effect orthermal transforming efficiency, etc. occurs and as a result, variousnonlinear absorption phenomena, for example, generation of plasma,further are induced. Thereby, high speed recording using pulse laserlight with a relatively small peak power can be achieved.

It is preferable that the pulse width of the recording light is in therange from 100 femtoseconds to 10 nanoseconds, and more preferably, inthe range from 1 picosecond to 100 picoseconds. In general, with theincrease in the pulse width of the laser light, the threshold value ofthe peak power for recording one bit on the information recording mediumtends to be reduced. This is thought because when the pulse width of thelaser light is increased, the energy is increased by the incrementthereof. By permitting the recording at a relatively low peak power, thestructure of the semiconductor laser that is a light source can besimplified. This is because the semiconductor laser with not so highpeak power is not likely to be broken in the end face in which light isemitted. Therefore, actually, in order to suppress the peak powerthreshold value to be about 3 kW or less, it is desirable that the pulsewidth is set to be 1 picosecond or more. On the other hand, in general,the energy threshold value necessary for recording of one bit is minimumwith respect to the laser light with the pulse width of severalpicoseconds. When the pulse width is increased, the energy thresholdvalue gradually tends to increase. Therefore, practically, in order tosuppress the energy threshold value to about 5 nJ or less, recordingwith a pulse width of 100 picoseconds or less is desired. Note here thatthe energy threshold value is obtained as a value by multiplying thepulse width of the laser light by the peak power threshold value.

Hereinafter, the case where the particles 11 are formed of zinc oxidewill be explained as an example.

FIG. 4 shows a spectral transmittance curve of an information recordingmedium, in which the recording portion 3 formed by laminating ten layersof 0.13 μm-thick recording layers 1 containing about 55 wt. % ofparticles 11 made of zinc oxide and about 45 wt. % of particle-holdingmaterial 10 made of polyester via intermediate layers 2, is disposedbetween the substrate 9 and the protective layer 4. Note here that theenergy gap of the zinc oxide is 3.2 eV and the absorption spectrumwavelength is 388 nm. As shown in FIG. 4, this information recordingmedium has a transmittance of about 90% including Fresnel reflectance ofthe surface of protective layer 4 and the surface of the substrate 9 ofabout 10% at the wavelength of 400 nm or more. Therefore, the recordinglayer 1 hardly absorbs and is transparent. Furthermore, in thisinformation recording medium, the transmittance is lowered rapidly atthe wavelength of 400 nm or less, and the transmittance becomes 0%,i.e., opaque at the wavelength of 370 nm. Therefore, in this informationrecording medium, it is possible to determine the predeterminedwavelength in the range of 400 nm or less in which the transmittance islowered rapidly. It is preferable to determine the predeterminedwavelength in which the absorption rate is about 5% or more per onelayer of the recording layers, and it is more preferable to determinethe predetermined wavelength around 370 nm in which the absorption rateis 100%. In this recording layer 1, in the case where the recording ofinformation by two-photon absorption is assumed, the light having twicethe wavelength of the predetermined wavelength is used as recordinglight and thus, it looks as if the focal point were irradiated with thelight with the predetermined wavelength. For example, when thepredetermined wavelength is determined at 370 nm, the recordingwavelength is set at 740 nm. Furthermore, the reproduction wavelength isset to be more than 400 nm in which the reproduction wavelength ispractically transparent in the recording layer. Since such recordinglight and reproducing light are hardly absorbed by particles, they arenot attenuated even when passing through the recording layer. Note herethat when recording by the multiphoton absorption such as n-photonabsorption, etc. is assumed, the wavelength of the recording light isset n-times of the predetermined wavelength and the same is true in theother cases.

As the particles 11, inorganic materials or semiconductor materials canbe used. The use of inorganic materials for the particles 11 enables therecording layer to be stable. Furthermore, the use of the semiconductormaterials for the particles 11 enables the transmittance of therecording layer in a certain range of wavelengths to be lowered rapidlyas shown in FIG. 4. This property is more suitable for recording usingthe non-linear absorption such as the two-photon absorption.Furthermore, in order to obtain high reproduction resolution, it ispreferable that the reproducing light having the wavelength of 150 nm to500 nm is used. By using semiconductor materials having an energy gap of2.5 eV or more and 8.3 eV or less for the particles 11, since thepredetermined wavelength can be set to be smaller than 500 nm, thereproducing wavelength can be set to be 500 nm or less. Thus, thereproduction performance can be improved, and higher density recordingcan be carried out. Examples of such semiconductor materials includezinc oxide (ZnO) having an energy gap of 3.2 eV (absorption spectrumwavelength of 388 nm); tin oxide (SnO₂) having an energy gap of 3.8 eV(absorption spectrum wavelength of 326 nm); zinc sulfide (ZnS) having anenergy gap of 3.6 eV (absorption spectrum wavelength of 344 nm);titanium oxide (TiO₂) having an energy gap of 3.2 eV (absorptionspectrum wavelength of 388 nm); tungsten oxide (WO₃) having an energygap of 3.2 eV (absorption spectrum wavelength of 388 nm); strontiumtitanate (SrTiO₃) having an energy gap of 3.2 eV (absorption spectrumwavelength of 388 nm); silicon carbide (SiC) having an energy gap of 3.0eV (absorption spectrum wavelength of 413 nm); indium oxide (In₂O₃)having an energy gap of 2.8 eV (absorption spectrum wavelength of 443nm); cadmium sulfide (CdS) having an energy gap of 2.5 eV (absorptionspectrum wavelength of 497 nm); and the like.

Next, a method for producing an information recording medium of thisembodiment will be explained with reference to FIGS. 2A to 2D.

Firstly, a substrate 9 is prepared (see FIG. 2A). On the substrate 9, arecording layer 1 a is formed by applying a coating containing theparticles 11 and the particle-holding material 10 by, for example, spincoating, etc. (see FIG. 2B). Furthermore, an intermediate layer 2 a isformed thereon by applying a coating containing materials for theintermediate layer by, for example, spin coating, etc. (see FIG. 2C).Furthermore, a recording layer 1 b, an intermediate layer 2 b, arecording layer 1 c, . . . a recording layer if are formed thereonrepeatedly, in the same manner. Finally, a protective layer 4 is formedby a method for applying a coating containing a material of theprotective layer 4 or a film formation method (see FIG. 2D). Thus, byforming the recording layer 1 and the intermediate layer 2 by applyingthe materials, an information recording medium of this embodiment can beproduced easily and at low cost.

Furthermore, by forming an excessive amount of the intermediate layer orthe recording layer, the excessive portion (i.e., a part at the sidewhere light is incident of the recording portion) may be formed as theprotective layer 4. That is to say, on the recording layer 1 f, a layerthat is the same as the intermediate layers 2 a to 2 e further is formedso as to form the protective layer 4, or by forming the recording layer1 f thick, and a part of the recording layer 1 f may be functioned as apart of the protective layer 4. Thus, it is not necessary to form theprotective layer 4 by a different process from the process for producingthe recording portion 3 and the protective layer 4 can be formed ofmaterials substantially the same as those of the recording portion.

Next, an optical information recording/reproducing apparatus of thisembodiment will be explained. As shown in FIG. 3, an optical head of theoptical information recording/reproducing apparatus of this embodimentis provided with two kinds of light sources 20 a and 20 b. In an opticalpath from the light sources 20 a and 20 b to the information recordingmedium 22, beam splitters 18 a and 18 b, a collimator lens 16, afocus/track error signal detection element 15, a mirror 121, a sphericalaberration correction element 13, and an objective lens 6 (lens pair 6 aand 6 b) are arranged. The light source 20 a is a semiconductor laserlight source having the wavelength of, for example, 405 nm and is usedfor reproducing information. The light source 20 b is a semiconductorpulse laser light source having the wavelength of, for example, 740 nmand the pulse width of, for example, 100 femtoseconds to 10 nanosecondsand is used for recording information.

At the time of recording, a laser light 21 b emitted from the lightsource 20 b is bent toward the y direction by the beam splitter 18 a; ismade into substantially parallel light by the collimator lens 16; istransmitted through a diffraction focus/track-error signal detectionelement 15 (using O-order diffraction light); and allows its opticalpath to bend toward the z direction by the mirror 121. Then, the laserlight 8 bent in the z direction, passes through a spherical aberrationcorrection element 13 and is focused on (as a convergent light 7) therecording portion 3 of the information recording medium 22 by theobjective lens 6. Thus the information bit 5 is formed as shown inFIG. 1. The information bit 5 is formed by the use of the change in theoptical constant of the recording layer 1. In this embodiment, theinformation bit 5 is formed by the use of the change in the refractiveindex or the change in the transmittance of the particle-holdingmaterial 10 of the recording layer 1, etc.

At the time of reproducing, a laser light 21 a emitted from the lightsource 20 a is transmitted through the beam splitters 18 a and 18 b; ismade into substantially parallel light by the collimator lens 16; istransmitted through a diffraction focus/track-error signal detectionelement 15 (using the 0-order diffraction light); and allows its opticalpath to bend toward the z direction by the mirror 121. Then, the laserlight 8 bent in the z direction passes through the spherical aberrationcorrection element 13 and is focused (as a convergent light 7) on therecording portion 3 of the information recording medium 22 by theobjective lens 6. The light reflected by the information bit 5 formed onthe recording layer 1 is bent in the opposite direction; passessequentially through the objective lens 6, the spherical aberrationcorrection element 13, and the mirror 121; is divided into a pluralityof lights by the diffraction focus/track-error signal detection element15 (first-order diffractive light is used. In FIG. 3, forsimplification, the divided light is not shown in the optical path fromthe diffraction focus/track-error signal detection element 15 to thebeam splitter 18 b) and is formed into the convergent light by thecollimator lens 16; and further deflected in the -z direction by thebeam splitter 18 b. A plurality of divided light 17 a to 17 c deflectedin the -z direction is transmitted through respective pin holes 14 a to14 c of a pin hole array 14, and then signals are detected by opticaldetectors 19 a to 19 c.

Note here that in the optical information recording/reproducingapparatus of this embodiment, the recording light source and reproducinglight source are provided separately. However, one light source isshared for the recording light source and the reproducing light source.In this case, the light source can be configured such that for example,the light source having, for example, the wavelength of 740 nm is usedand in recording, the laser light having a large peak power is emittedvia pulse oscillation, and in reproducing, laser light having the smallpeak power is emitted via successive oscillation. Thus, the number oflight sources is one and the configuration of the apparatus can besimplified.

Furthermore, in this embodiment, a pin-hole array 14 composed of aplurality of pin-holes is placed substantially at the focal point of theentire divided light 17 a to 17 c. However, separate pin-holes may beplaced corresponding to the respective focal points. The sizes of thepin-holes 14 a to 14 b are made to be smaller than the respectiveconvergent light 17 a to 17 c, whereby light only in the central portionof the convergent light 17 a to 17 c can be detected and unnecessaryhigh-order aberration light distributed around the periphery of theconvergent light 17 a to 17 c can be removed. Thus, S/N of not onlyreproducing signals but also servo error signals can be improved. Notehere that when the light amount is reduced by removing the peripherallight of the divided light 17 a to 17 c, it is preferable that thesignal intensity is increased by using APD (avalanche photodiode) for aphotodetector 19. Note here that in the case of the informationrecording medium in which a plurality of recording layers are provided,a large amount of detected light cannot be obtained because of thelimitation of the material. Also for this reason, it is preferable touse APD.

Furthermore, instead of the pin-hole array 14, divided light 17 a to 17c may be detected respectively by the photodetectors 19 a to 19 c whoseareas are smaller than the respective divided light 17 a to 17 c. Inthis case, the same effect can be obtained.

Furthermore, only the divided light 17 b and 17 c corresponding to thetrack error signals are allowed to pass through the pin-holes 14 b and14 c of the pin-hole array 14 and detected at the photodetector 19 b and19 c, and the divided light 17 a corresponding to the focus error signalis not allowed to pass through the pin-hole and detected directly by,for example, a four-dividing photodetector 19 a. With such anarrangement, as the focus detection method, for example, an astigmatismmethod can be used. Furthermore, when the area of the photodetector 19 acan be made to be smaller than the cross-sectional area of the dividedlight 17 a at the detection point, the high-order aberration componentcan be reduced.

In this embodiment, by configuring the objective lens 6 as a lens pair,the numerical aperture (NA) is increased (for example, NA is 0.85). Notehere that also in the case where the objective lens 6 is a single lens,the numerical aperture can be increased.

In this embodiment, the information bit 5 is recordedthree-dimensionally on the recording portion 3 sequentially in the orderin which the recording light does not pass through the information bit 5that has already been recorded. By recording in such an order, it ispossible to reduce stray light such as scattered light and unnecessarydiffracted light from the information bit 5. Specifically, by formingthe information bit 5 in the order from the recording layer located atthe most distant from the objective lens 6 (recording layer 1 a inFIG. 1) to the nearest recording layer, the recording in theabove-mentioned order can be possible. In the information recordingmedium shown in FIG. 1, information is recorded three dimensionally inthe -z direction in the order from the recording layer 1 a, therecording layer 1 b, the recording layer 1 c . . . . At this time, sincethe thickness of the recording layer 1 through which the convergentlight 7 passes is different depends upon the recording depth of theinformation bit 5, it is preferable that recording is carried out whilecontrolling the amount of spherical aberration in accordance with therecording depth by the spherical aberration correction element 13provided in the optical path from the light sources 20 a and 20 b to theobjective lens 6. Thus, an excellent information bit 5 can be formed.For the spherical aberration correction element 13, a liquid crystalelement capable of changing the refractive index distribution, a beamexpander, in which a concave lens and a convex lens are combined and theinterval between both lenses in the direction of an optical axis can bechanged by an actuator, and the like can be used.

Note here that the recording order is not always limited to the -zdirection as long as the convergent light 7 does not pass through theinformation bit 5 that has already been recorded if unrecorded portionsof the information bit 5 are present.

Embodiment 2

An information recording medium and a method for producing the sameaccording to Embodiment 2 of the present invention will be explainedwith reference to the drawings. FIGS. 5A and 5B show a cross-sectionalconfiguration of the information recording medium and a state in whichinformation is recorded and reproduced in the embodiment of the presentinvention.

As shown in FIG. 5A, in the information recording medium of thisembodiment, on a substrate 39, a recording portion 33 and a protectivelayer 34 are formed. In the recording portion 33, a plurality oflaminated bodies each composed of a recording layer and an auxiliaryrecording layer (a particle-containing layer) are laminated viaintermediate layers. That is to say, in the recording portion 33, fromthe side of the substrate 39, an auxiliary recording layer 37 a, arecording layer 31 a, an intermediate layer 32 a, an auxiliary recordinglayer 37 b, a recording layer 31 b, an intermediate layer 32 b, . . . ,an intermediate layer 32 e, an auxiliary recording layer 37 f and arecording layer 31 f are laminated sequentially. In the informationrecording medium of this embodiment, by including a plurality ofrecording layers in the recording portion 33, in addition totwo-dimensional information recording, three-dimensional recordingcapable of recording by superimposing a plurality of layers in thethickness direction can be carried out. Note here that hereinafter, anarbitrary recording layer in the recording layers 31 a to 31 f will bereferred to as a recording layer 31; an arbitrary intermediate layer inthe intermediate layers 32 a to 32 e will be referred to as anintermediate layer 32; and an arbitrary auxiliary recording layer in theintermediate layers 37 a to 37 e will be referred to as an auxiliaryrecording layer 37.

FIG. 5B is an enlarged view showing the recording layer 31 and theauxiliary recording layer 37 (in the region 36 shown in FIG. 5A). Asshown in FIG. 5B, the auxiliary recording layer 37 is provided incontact with the recording layer 31 and the auxiliary recording layer 37is formed of the particle-holding material 40 in which particles 41 aredispersed. The particles 41 have the same properties and functions andcan be formed of the same material as the particles 11 contained in therecording layer 1 of the information recording medium explained inEmbodiment 1. Furthermore, also the particle-holding material 40 isrequired to be substantially transparent to the recording light and thereproducing light similar to the particle-holding material 10 containedin the recording layer included in the information recording medium inEmbodiment 1. However, the optical constant of the particle-holdingmaterial 40 does not necessarily change at the predetermined temperatureand it is rather preferable that the optical constant does not change.This is because in the information recording medium of this embodiment,the particle-holding material 40 does not function as a recordingmaterial but recording is carried out on the recording layer 31.

As the recording layer 31, a material whose refractive index ortransmittance changes at the predetermined temperature is used, and aresin, tellurium glass, chalcogenide glass, and the like can be used.However, since a plurality of the recording layers are laminated to forma multi-layered structure, in order to reduce the loss of light, it ispreferable that the recording layer is substantially transparent to therecording light and the reproducing light. The substrate 39, theprotective layer 34 and the intermediate layer 32 have the samefunctions as those of the substrate 9, the protective layer 4 and theintermediate layer 2 respectively and can be formed of the samematerials.

Next, recording information on the information recording medium of thisembodiment will be explained.

In the information recording medium of this embodiment, like therecording layer 1 of the information recording medium of Embodiment 1,at the focal point of the recording light, a nonlinear absorptionphenomenon such as the two-photon absorption is induced in the auxiliaryrecording layer 37 and the particles 41 absorb the recording light. Theparticles 41 absorb light and generate heat, and the heat is transferredto the recording layer 31 provided in contact with the auxiliaryrecording layer 37, thus increasing the temperature of the recordinglayer 31. When the temperature of the recording layer 31 reaches thepredetermined temperature, the optical constant of the recording layer31 changes and the information bit 35 is formed. According to thisinformation recording medium, the recording sensitivity is improvedversus that of the conventional information recording medium. This isthought because light is absorbed by the particles 41 contained in theauxiliary recording layer 37, the effect unique to the particles, thatis, excellent quantum effect or thermal transforming efficiency, etc.occurs and as a result, various nonlinear absorption phenomena, forexample, further are induced.

For the same reason in Embodiment 1, it is preferable that the recordinglight has a peak power of several tens to several hundreds mW or moreand several kW or less, and the pulse width of in the range from 100femtoseconds to 10 nanoseconds (more preferably, in the range from 1picosecond to 100 picoseconds).

Also in the method for producing the information recording medium ofthis embodiment, it is preferable that respective layers are formed bycoating the materials sequentially as in Embodiment 1. It isadvantageous because the information recording medium can be producedeasily and at low cost.

Furthermore, in the case where information is recorded and reproducedwith respect to the information recording medium of this embodiment, theoptical information recording/reproducing apparatus shown in FIG. 3 canbe used.

Embodiment 3

An information recording medium and a method for producing the sameaccording to Embodiment 3 of the present invention will be explainedwith reference to drawings. FIGS. 6A and 6B show a cross-sectionalconfiguration of the information recording medium and a state in whichinformation is recorded/reproduced in the embodiment of the presentinvention.

As shown in FIG. 6A, in the information recording medium of thisembodiment, on a substrate 49, a recording portion 43 and a protectivelayer 44 are formed. The information recording medium of this embodimentis different from the information recording media according toEmbodiments 1 and 2 in that the whole recording portion 43 functions asa recording layer (particle-containing layer) 41. On substantially thesame plane in the recording layer 41, rows of the information bits 45are recorded. By providing a plurality of such recording surfaces (41 ato 41 f) in the recording layer 41, information can be recordedthree-dimensionally.

FIG. 6B is an enlarged view showing the recording layer 41 (in theregion 46 shown in FIG. 6A). As shown in FIG. 6B, the recording layer 41is formed of the particle-holding material 50 in which particles 51 aredispersed. The particles 51 have the same properties and functions asthose contained in the recording layer 1 of the information recordingmedium explained in Embodiment 1 and can be formed of the same materialsas those in Embodiment 1. However, in the information recording mediumof this embodiment, it is preferable that the content of the particles51 is in the range from 0.3 wt. % to 10 wt. %. This is because it ispossible to improve the recording sensitivity when the content is 0.3wt. % or more, and it is possible to suppress the scattering loss so asto increase the light utilization efficiency when the content is 10 wt.% or less. The particle-holding material 50 has the same properties andfunctions as those of the particles-holding material 10 contained in therecording layer 1 of the information recording medium shown inEmbodiment 1 and can be formed of the same materials as in Embodiment 1.In the case where the substrate 49 and the protective layer 44 areformed of the same material, the structure is simplified further and canbe produced at low cost.

Recording of information on the information recording medium of thisembodiment is the same as in Embodiment 1, therefore, for the samereason of Embodiment 1, it is preferable that the recording light has apeak power of several tens to several hundreds mW or more and several kWor less, and the pulse width of in the range from 100 femtoseconds to 10nanoseconds (more preferably, in the range from 1 picosecond to 100picoseconds).

The information recording medium in this embodiment can be produced byapplying a coating containing the particles 51 and the particle-holdingmaterial 50 on the substrate 49, or by forming the recording layer 41 byinjection molding. With such methods, the information recording mediumcan be formed easily at low cost.

Furthermore, in the case of reproducing information with respect to theinformation recording medium of this embodiment, it is possible to usean optical information recording/reproducing apparatus shown in FIG. 3.

In Embodiments 1 to 3 mentioned above, the embodiments of the presentinvention are explained. However, the present invention is notnecessarily limited to these embodiments. It is possible to combineinformation recording media and the production methods therefof, and tocombine the configurations of the optical informationrecording/reproducing apparatuses of the embodiments. Thus, the sameeffects can be obtained. Furthermore, the information recording mediumand the optical information recording/reproducing apparatus of thepresent invention may include a rewritable type apparatus in addition toa write-one-type apparatus.

Furthermore, in Embodiments 1 and 2, the information recording mediumincluding six layers of recording layers was explained. However, thenumber of laminating is not limited to six, and the number of therecording layers to be laminated layers is in the range of two or moreand 100 layer or less.

Furthermore, in Embodiments 1 to 3, an optical disk was described as anexample of the information recording medium, but the present inventionis not limited to this alone, and the optical disk of the presentinvention may include a card type, drum type, and tape type media.

Note here that the objective lens and the collimator lens mentioned inthe above-mentioned embodiment are mentioned for convention and they arethe same as general lenses.

Example

Next, one Example of the information recording medium of this embodimentwill be explained. The information recording medium of this Example isan example of the information recording medium as shown in FIG. 1explained in Embodiment 1.

On a polycarbonate substrate 9 (thickness: 1.1 mm) on the surface ofwhich track grooves (groove for groove-recording having a pitch of 0.32μm and the depth of 30 nm) were formed, a plurality of recording layerseach having a thickness of 0.13 μm to 1 μm and intermediate layers eachhaving a thickness of 3 μm were laminated alternately by spin coating soas to form a recording portion 3 (recording layers 1 a to if andintermediate layers 2 a to 2 e), and further polycarbonate protectivelayer 4 (thickness: 100 μm) was formed. The recording layers 1 a to ifwere formed of particles 11 including 55 wt. % of zinc oxide (averageparticle-size of 0.03 μm) and a particle-holding material 10 containing45 wt. % of an ultraviolet curable resin. Furthermore, the intermediatelayers 2 a to 2 e were formed of an ultraviolet curable resin.

The spectral transmittance curve of the information recording medium ofthis Example was substantially the same as that shown in FIG. 4. Then,the predetermined wavelength was set to 370 nm and the recordingwavelength was set to 740 nm.

FIG. 7 shows the measurement results of the energy threshold value andthe peak power threshold value at the time of one bit recording withrespect to the information recording medium when the pulse width waschanged and irradiation with pulse light with the wavelength of 740 nmwas carried out. With the increase in the pulse width of the pulse laserlight, the threshold value of the peak power for recording one bit onthe information recording medium tends to be lowered. By setting thepulse width to be one picosecond or more, the peak power threshold valuecould be suppressed to 3 kW or less. On the other hand, the energythreshold value required for one-bit recording was a minimum withrespect to laser light with the pulse width of several picoseconds. Withthe increase in the pulse width, the energy threshold value tends to beincreased gradually. By setting the pulse width to 100 picoseconds orless, the energy threshold value could be suppressed to 5 nJ or less.

From the results mentioned above, according to the information recordingmedium of this Example, by using the laser light with the peak power of3 kW or less, it was confirmed that one information bit was formed byone pulse.

Industrial Applicability

According to the information recording medium and a method for producingthe same, and an optical information recording/reproducing apparatus ofthe present invention, the recording sensitivity is improved, andtherefore one information bit can be formed by one pulse even if laserlight having extremely high peak power is not used as conventionally.Therefore, it is possible to provide an information recording mediumcapable of recording with high sensitivity and at high speed, and anoptical information recording/reproducing apparatus.

1. An information recording medium comprising a recording portioncapable of recording information three-dimensionally, wherein therecording portion comprises at least one particle-containing layercomprising: particles that absorb at least a part of light with apredetermined wavelength; are substantially transparent to recordinglight and reproducing light with wavelengths longer than thepredetermined wavelength; and have an absorption rate with respect tothe light with the predetermined wavelength being higher than theabsorption rate with respect to the recording light and the reproducinglight, and a particle-holding material that is substantially transparentto the recording light and the reproducing light.
 2. The informationrecording medium according to claim 1, wherein the average particle-sizeof the particles is shorter than the wavelength of the recording lightand the wavelength of the reproducing light.
 3. The informationrecording medium according to claim 2, wherein the average particle-sizeof the particles is shorter than a quarter of the wavelength of therecording light and a quarter of the wavelength of the reproducinglight.
 4. The information recording medium according to claim 1, whereinthe particle-containing layer is a recording layer, and theparticle-holding material has an optical constant that changes at apredetermined temperature.
 5. The information recording medium accordingto claim 4, wherein the recording portion comprises a plurality of therecording layers, and the plurality of recording layers are laminatedvia intermediate layers that are substantially transparent to therecording light and the reproducing light.
 6. The information recordingmedium according to claim 1, wherein the recording portion furthercomprises a recording layer, and the particle-containing layer is anauxiliary recording layer disposed in contact with the recording layer.7. The information recording medium according to claim 6, wherein aplurality of laminated bodies composed of the recording layer and theauxiliary recording layer are provided and between the laminated bodiesthat are adjacent to each other, an intermediate layer that issubstantially transparent to the recording light and the reproducinglight is provided.
 8. The information recording medium according toclaim 1, wherein the particle-containing layer is a recording layer; theparticle-holding material has an optical constant that changes at apredetermined temperature; and the whole part of the recording portionis made of the one recording layer.
 9. The information recording mediumaccording to claim 1, wherein a protective layer that is substantiallytransparent to the recording light and the reproducing light is providedat the side of the recording portion on which light is incident
 10. Theinformation recording medium according to claim 9, wherein theprotective layer is made of the same material as that of theparticle-holding material.
 11. The information recording mediumaccording to claim 5, wherein the intermediate layer is made of the samematerial as that of the particle-holding material.
 12. The informationrecording medium according to claim 1, wherein the difference betweenthe refractive index of the particle-holding material and the refractiveindex of the particles is 0.5 or less.
 13. The information recordingmedium according to claim 1, wherein the particles comprise inorganicmaterials.
 14. The information recording medium according to claim 1,wherein the particles comprise semiconductor materials.
 15. Theinformation recording medium according to claim 14, wherein thesemiconductor material has an energy gap of 2.5 eV or more and 8.3 eV orless.
 16. The information recording medium according to claim 15,wherein the particles comprise at least one selected from the groupconsisting of zinc oxide, tin oxide, zinc sulfide, titanium oxide,tungsten oxide, strontium titanate, silicon carbide, indium oxide, andcadmium sulfide.
 17. The information recording medium according to claim5, wherein the particle-containing layer comprises 1 wt. % or more and95 wt. % or less of the particles.
 18. The information recording mediumaccording to claim 8, wherein the particle-containing layer comprises0.3 wt. % or more and 10 wt. % or less of the particles.
 19. Theinformation recording medium according to claim 1, wherein theparticle-holding material is a resin.
 20. A method for producing theinformation recording medium described in claim 1, the methodcomprising: forming a coating containing particles and aparticle-holding material; and applying the coating so as to form aparticle-containing layer.
 21. The method for producing the informationrecording medium according to claim 20, further comprising: forming anintermediate layer by applying the coating containing materialssubstantially transparent to recording light and reproducing light,wherein in the step of forming the particle-containing layer, a materialhaving an optical constant that changes at a predetermined temperatureis used as the particle-holding material, and the step of forming theparticle-containing layer and the step of forming the intermediate layerare repeated alternately predetermined number of times.
 22. The methodfor producing the information recording medium according to claim 20,further comprising: forming an intermediate layer by applying a coatingcontaining a material that is substantially transparent to the recordinglight and the reproducing light; and forming a recording layer byapplying a coating containing a material having an optical constant thatchanges at a predetermined temperature; wherein the step of forming theparticle-containing layer, the step of forming the recording layer, andthe step of forming the intermediate layer are repeated in apredetermined order and predetermined number of times periodically. 23.An optical information recording/reproducing apparatus for recording andreproducing information with respect to the information recording mediumdescribed in claim 1, the apparatus comprising: a light source foremitting recording light; a light source for emitting reproducing light;an objective lens for focusing the light emitted from the light sourceson the information recording medium; and a photodetector for detectinglight reflected by the information recording medium; wherein by usingthe change in the optical constant of a recording portion of theinformation recording medium, an information bit is recorded on therecording portion three-dimensionally.
 24. The optical informationrecording/reproducing apparatus according to claim 23, wherein the lightsource for emitting recording light is a pulse laser light source, and apulse width is in the range from 100 femtoseconds to 10 nanoseconds. 25.The optical information recording/reproducing apparatus according toclaim 24, wherein the pulse width is in the range from 1 picosecond to100 picoseconds.
 26. The optical information recording/reproducingapparatus according to claim 23, wherein the wavelength of the lightsource for emitting reproducing light is shorter than the wavelength ofthe light source for emitting recording light.
 27. The opticalinformation recording/reproducing apparatus according to claim 23,wherein the information bit is recorded on the recording portion of theinformation recording medium by using a nonlinear absorption phenomenon.28. The optical information recording/reproducing apparatus according toclaim 27, wherein the nonlinear absorption phenomenon comprises thetwo-photon absorption or multiphoton absorption.
 29. The opticalinformation recording/reproducing apparatus according to claim 23,wherein the information bit is recorded three-dimensionally on therecording portion of the information recording medium in an order inwhich the recording light does not pass through information bits thatalready have been recorded on the recording portion.
 30. The opticalinformation recording/reproducing apparatus according to claim 29,wherein the information bit is recorded sequentially in the order fromthe position distant from the objective lens to the position near theobjective lens in the recording portion of the information recordingmedium.
 31. The optical information recording/reproducing apparatusaccording to claim 23, wherein one light source is shared for the lightsource for emitting the recording light and the light source foremitting the reproducing light.
 32. The information recording mediumaccording to claim 7, wherein the intermediate layer is made of the samematerial as that of the particle-holding material.
 33. The informationrecording medium according to claim 7, wherein the particle-containinglayer comprises 1 wt. % or more and 95 wt. % or less of the particles.